A known method for straightening curly or frizzy hair involves the use of straightening irons. The high temperature of the iron leads to a breakage of hydrogen bonds in the keratin of the hair, achieving a temporary straightening. The hydrogen bonds are formed again by the action of moisture, so that the hair reverts back to its original shape over the time because of air humidity, and the straightening effect vanishes after washing the hair.
The shape of the hair is largely determined by the disulfide bonds linking two cysteine moieties of the hair keratin. In order to achieve a more permanent shaping of the hair, known methods involve the cleavage of the disulfide bonds by the action of a sulfide- or thio group containing reducing agent. After the hair has been brought into the desired shape, new disulfide bonds are formed by applying an oxidizing agent such as hydrogen peroxide, thus fixing the shape of the hair. The use of such agents, however, may cause damage to the hair.
As an example for this kind of hair shaping treatment, reference is made to GB 1 416 564, describing reducing compositions comprising thioglycolates or thiolactates as reducing agents and fixing compositions comprising hydrogen peroxide as an oxidizing agent. The reducing compositions may further comprise a salt of an acid such as glyoxylic acid as a buffering agent.
As an alternative to the above-described two-step reduction and oxidation process, the disulfide bridges can be cleaved by the action of an alkaline agent such as sodium hydroxide at a pH of about 11 or higher. Under these conditions, the disulfide (or cystin) moiety can undergo a disproportionation reaction under the elimination of sulfur, and is cleaved into an alpha-beta-unsaturated dehydro-alanine moiety and a cysteine moiety. After the hair has been brought into the desired shape, the dehydro-alanin moieties and the cysteine moieties form thioether bonds and combine to lanthionine, stabilizing the straightened state of the hair. Since the disulfide or cystin moieties are converted into lanthionine moieties, this type of hair straightening process using an alkaline agent is also called lanthionization.
Both the two-stage reduction/oxidation method and the lanthionization method rely on a cleavage of the disulfide bonds and the formation of new bonds among the hair proteins, leading to an irreversible change of the shape of the hair. This means that these processes can achieve a permanent straightening, wherein the treated portion of the hair maintains its shape, and the straightening effect only vanishes because of the growth of the hair.
Recently, it has been found that carboxylic acids having a carbonyl group adjacent to the carboxy group, such as glyoxylic acid, which are known as a buffering agent in cosmetic compositions, may have a semi-permanent straightening effect when used in combination with mechanical straightening means.
In this respect, WO 2011/104282 describes a process for semi-permanent hair straightening, which involves applying a composition comprising an α-keto acid onto the hair, leaving the composition in contact with the hair for 15 to 120 minutes, drying the hair and straightening the hair with a straightening iron at a temperature of 200±50° C.
Furthermore, WO 2012/010351 describes a treatment for semi-permanent straightening of curly, frizzy or wavy hair by applying a solution of glyoxylic acid in combination with mechanical straightening, using a straightening iron at a temperature of 200±30° C. After the treatment, the hair is said to retain its shape for at least six consecutive washings.
EP 0685 219 describes a composition useful as a pre-treatment agent before subjecting the hair to a perm wave treatment, the composition comprising a film-forming cationic, anionic and/or amphoteric polymer, an amphoteric surfactant, an organic acid and a C2 to C4 alcohol, and being free of cationic surfactants.